This invention relates to the low temperature treatment of water containing solid substances for the purpose of preserving them against deterioration and more particularly to a method and apparatus determining the low temperature characteristics of such substance.
The deterioration of a wide range of solid substances, both organic and inorganic, such as food products, biological materials to be stored for medical and/or scientific purposes, e.g. fragments of animal organs and tissue; as well as various extract substances, such as serums, hormones, vitamins, and the like, such substances being included in a wide range of solid substances, both organic and inorganic, is primarily ascribable to the presence of free water and of water bonded by way of labile bonds in the molecular structure. As a result, considerable effort has been expended in an attempt to extract such water by a process that would not in turn be the cause of some irreversible effect conducive to deterioration of the treated substance.
Attempts involving direct desiccation of the substances have failed since the direct conversion of liquid water into water vapor, as by evaporation at ordinary ambient temperature in a vacuum, results in deleterious, irreversible modifications in the texture of the process materials, regardless of the precautions taken. As a result, attempts such as these have been abandoned and other methods have been investigated.
One such method, which has yielded generally satisfactory results, involves subjecting the sample to a freezing treatment in order to freeze the water as ice in situ and thereby impart a high rigidity to the physicochemical structure of the material. However, in order to preserve the frozen sample, it must be continually exposed to very low temperatures, thereby detracting from the practical value of such methods, especially in those cases where the frozen products have to be transported over long distances, or where they must be stored in quantity for long periods of time.
Another method, variously known as freeze-drying, cryo-desiccation, or lyophilization, has been developed and has yielded excellent results. This method essentially comprises a low temperature dehydration of the frozen samples by direct sublimation, under reduced pressure, of the ice in the sample. This process involves the direct conversion of the water from the solid to the vapor phase, there being an absence of liquid water. After the dehydrating step, the substance may be conveniently stored in a dry atmosphere at ordinary ambient temperatures. In order to restore the substance to its pristine form, it is only necessary to reintroduce the same amount of water into the substance as was removed during the process.
In conventional lyophilization apparatus, the ice sublimating step is generally performed in an enclosure in which a vacuum can be maintained and in which the extracted vapor is condensed on a cold surface. Since the sublimation of ice is highly endothermic (absorbing 650 calories per gram at -30.degree. C.), the temperature of the material tends to drop during the process, thereby reducing the rate at which sublimation proceeds. Consequently, in order to avoid excessive operating times, heating means must be provided which will supply heat to the material in amounts substantially equivalent to that removed by sublimation.
In prior art lyophilizing apparatus, the operation of the apparatus had been monitoring and/or controlled in terms of temperature measurement. In some apparatus, means are provided for obtaining some prescribed law of temperature variation of the product vs. time throughout the process. The methods of regulating such processes on the basis of temperature measurements suffer from an inherent disadvantage in that they do not provide any means of appreciating the actual structural modifications occuring within the processed substance during treatment at low temperatures. All that is provided is an overall indication of temperature which does not make it possible to discriminate between temperature in the dehydrated portion and temperature in the frozen portion within a given sample. In addition, temperature is not a reliable reference whereby to determine the structure of the material due to various and complex physico-chemical phenomenon such as supercooling and the like, which considerably modifies the matrix structure.
In view of the above, it has in the past been necessary to perform a number of preliminary tests which serve to evaluate the particular values to be used for the operating factors of the regulating system for each particular group of samples to be treated. In addition, a large safety margin had to be allowed for in the selection of such values if a partial or complete deterioration of materials was to be avoided. Since the actual treating temperature was thus considerably lower than the optimum temperature, the sublimation rate and hence the output rate of the process were correspondingly reduced substantially. If, on the other hand, in an attempt to increase the production rate, the operating temperature was maintained close to or equal to its highest permissible value, the product yield, meeting specifications, could be greatly reduced.
Another method of regulation or monitoring, based on a measurement of the vapor pressure present within the frozen portions and the fluid portions of the material, has been used to avoid the above stated difficulties. This method however requires the use of complex apparatus having considerable response delay or inertia. In addition, the apparatus is inherently incapable of leading to fully successful results since the only portion of the sample accessible to the measurements required is that area lying close to the interface between the dry and the frozen portions of the substance, which interface gradually advances deeper and deeper into the sample as the treatment progresses. The degree of reliable protection afforded by such monitoring methods, is therefore necessarily limited.
It has also been proposed to determine and regulate the quantity of cold units or "frigories" (negative calories) remaining available in a refrigerator, using a eutectic solution as a refrigerating medium therein, by means including a pair of electrodes contacting the eutectic solution and connected in a circuit with indicating or regulating output means, and with a DC or preferrably AC energy supply. In such systems, the measuring and/or regulating operation is based on the fact that the eutectic solution, on passing from the solid to the liquid state or vice versa, sustains a substantial change in electrical resistivity and such resistivity change is used to generate an electrical signal controlling the indicator or regulator means.
A method of controlling or monitoring the freezing and/or freeze drying operation on water containing solid substances, relating to the use of the electrical resistivity parameter, is disclosed in U. S. Pat. No. 3,078,586, issued to L. P. Rey on Feb. 26, 1963. The method of Rey consists of continually surveying, throughout the operation, the variations of a selected electric characteristic, e.g. resistivity or dielectric constant, of a sample of the substance itself being processed and eventually controlling the temperature applied to the substance so as to maintain the characteristics at a desired value at all times throughout the process. One deficiency of the method of Rey occurs when the substance is at or near the freezing point. Under these conditions, resistivity alone is not a sufficiently precise indicator to permit the process to be carried out with maximum efficiency, i.e. permitting the highest product yield in the shortest amount of time. Furthermore, resistivity alone does not provide sufficient information regarding endothermic or exothermic reactions that may result from changes in phase or structure at the freezing point of the substance.
At present, the thermal and electrical properties as a function of temperature are normally examined in separate apparatuses, and, in some cases, using different fill volumes and containers. Also, the standard differential thermal analysis equipment is not designed to accomodate a glass container that may have a volume of 100 ml. In addition, as indicated above, the variation of the magnitude of resistivity or resistance as a function of temperature is not a sharp enough indicator to distinguish the presence of mobile water. In addition, in the case of low fill volumes, the resistance of the measuring cell may be outside the upper resistance range of the instrument.